1. Field of the Invention
The present invention relates to the preparation of hop acid derivatives for flavoring beer and other beverages, as well as for use in non-brewing applications. More particularly, it relates to a method of preparing tetrahydroiso-.alpha.-acids.
2. Background of the Art
Tetrahydroiso-.alpha.-acids are a light stable bittering agent which can be prepared from either .alpha.-acids or .beta.-acids. .alpha.-acids and .beta.-acids are two major components which are present in extracts of whole hops. Tetrahydroiso-.alpha.-acids can be prepared from .alpha.-acids by isomerizing the .alpha.-acids to iso-.alpha.-acids followed by the hydrogenation of iso-.alpha.-acids to tetrahydroiso-.alpha.-acids. Tetrahydroiso-.alpha.-acids can also be prepared by the hydrogenolysis/hydrogenation of .beta.-acids to 4-deoxytetrahydro-.alpha.-acids followed by the oxidation/isomerization of 4-deoxytetrahydro-.alpha.-acids to tetrahydroiso-.alpha.-acids.
U.S. Pat. Nos. 3,552,975; 3,923,897; and 4,644,084 disclose the preparation of tetrahydroiso-.alpha.-acids from .beta.-acids. The disclosures of these patents, and all other patents and publications cited hereinafter, are hereby incorporated by reference as if fully set forth herein. However, such processes using .beta.-acids as a starting material require costly and time consuming multi-step manipulations or the use of objectionable organic solvents and chemicals.
U.S. Pat. Nos. 5,013,571; 5,296,637; 5,523,489; 5,600,012; 5,767,319; and 5,874,633 disclose the preparation of tetrahydroiso-.alpha.-acids utilizing .alpha.-acids or iso-.alpha.-acids as the starting material.
U.S. Pat. No. 5,013,571 discloses the simultaneous isomerization/hydrogenation of .alpha.-acids to tetrahydroiso-.alpha.-acids in a protic solvent system at a pH of 8-12. This patent also discloses the hydrogenation of iso-.alpha.-acids to tetrahydroiso-.alpha.-acids in a protic reaction system at a pH of 5-12.
U.S. Pat. No. 5,296,637 discloses the hydrogenation of .alpha.-acids to tetrahydro-.alpha.-acids followed by the isomerization of tetrahydro-.alpha.-acids to tetrahydroiso-.alpha.-acids.
U.S. Pat. No. 5,523,489 discloses the preparation of tetrahydroiso-.alpha.-acids from iso-.alpha.-acids by the hydrogenation in a reaction solvent of ethanol containing up to 15% water (w/w) at a pH of 1-7.
U.S. Pat. No. 5,600,012 discloses a process for the preparation of tetrahydroiso-.alpha.-acids by hydrogenating iso-.alpha.-acids in a non-buffered alcohol solution.
U.S. Pat. No. 5,767,319 discloses the hydrogenation of iso-.alpha.-acid metal salts to tetrahydroiso-.alpha.-acids in an aqueous lower alkanol solvent having a pH greater than 5.
U.S. Pat. No. 5,874,633 discloses a process of making concentrated single phase solutions of tetrahydroiso-.alpha.-acids by hydrogenating a concentrated aqueous alkaline solution of iso-.alpha.-acids in an alkanol solvent at a pH of at least above 6.
Anteunis, et al., Bull. Soc. Chim. Belq. 68, pp. 476-483 (1959) disclose the catalytic hydrogenation of .alpha.-acids in methanol or ethanol under various pH regimes.
Brown, et al., J. Chem. Soc. 59, pp. 545-551 (1959) disclose the catalytic hydrogenation iso-.alpha.-acids in methanol.
Section 172.250 (b) (6) of Title 21 of the Code of Federal Regulations discloses the hydrogenation of hop extracts using a palladium catalyst in methanol, ethanol, or isopropanol acidified with hydrochloric or sulfuric acid.
With regard to the hydrogenation of iso-.alpha.-acids in low pH reaction solutions (as in, for example U.S. Pat. Nos. 5,013,571 and 5,523,489), we have found that if reaction times and temperatures are not closely monitored, the conversion of tetrahydroiso-.alpha.-acids into over-reduced products can be considerable.
For practical and economic reasons, the hydrogenation of iso-.alpha.-acids should be relatively simple to operate and result in products of high purity and yield. Any deviation in hydrogenation conditions can cause an incomplete reaction which produces edihydroiso-.alpha.-acids (partial hydrogenation) along with unreduced iso-.alpha.-acids. These products cause light instability in beer. Also, an excessive hydrogenation (perhydrogenation) can occur which gives rise to over-reduced products, called neo-tetrahydroiso-.alpha.-acids. Such over-reduced products do not add bitterness to beer.
A complete hydrogenation usually is not achieved without considerable efforts in choosing a selective catalyst, varying the catalyst loading, manipulating the reaction time, changing the reaction temperature and precise monitoring the formation of the products. Quite often, over-reduced products are obtained due to lack of a clear indication to terminate the hydrogenation reaction at the stage where the two double bonds (C.dbd.C) have been completely reduced. In particular, when the process is scaled up, inconsistent hydrogenation and over-reduction often occur. Further, the hydrogenation of iso-.alpha.-acids to tetrahydroiso-.alpha.-acids using only a palladium on carbon type catalyst (due to the BATF regulation) and no organic solvents other than alcohol (when used in food applications) is very delicate and difficult to precisely control. Development of a rugged process is vital in order to achieve a successful scale-up.
Also, such catalytic hydrogenation processes typically require the recovery and refabrication of spent hydrogenation catalyst. Such recovery and refabrication involves the recapture of the noble metal in the spent catalyst followed by the reimpregnation of the noble metal on a suitable support (like carbon). The cost of spent catalyst recovery and refabrication includes not only the direct costs for the same but also the inevitable loss of a certain amount of noble metal (like palladium) during the recovery and refabrication of the catalyst.
The prior art discussed above suffers from one or more of the aforementioned problems. It obviously would be desirable to have a simple method of making tetrahydroiso-.alpha.-acids from iso-.alpha.-acids which does not result in the formation of undesired products and in which it is easy to isolate the tetrahydroiso-.alpha.-acids from the reaction mixture. It would also be desirable to recycle at least a portion of the spent hydrogenation catalyst to reduce the cost of recovering and refabricating such catalyst.